The present invention relates to an exposure control drive apparatus.
Conventionally, an actuator such as a motor or the like for driving a member for controlling an exposure device such as a shutter or a diaphragm is driven by using a battery power source via a regulator as shown by FIG. 14.
FIG. 14 shows an example of a conventional digital still camera. In the drawing, a regulator REG1 stably supplies a regulated output voltage of a battery E to a control circuit (hereinafter, referred to as xe2x80x9cCPUxe2x80x9d) 100, a light measuring circuit (hereinafter, referred to as xe2x80x9cAExe2x80x9d) 2, a distance measuring circuit (hereinafter, referred to as xe2x80x9cAFxe2x80x9d) 3, a photographing unit (hereinafter, referred to as xe2x80x9cCCDxe2x80x9d) 4 including CCD(s) as photographing elements, a CCD drive circuit (hereinafter, referred to as xe2x80x9cCCDDRxe2x80x9d), an image signal processing circuit and a drive voltage generating circuit and an electronic viewfinder (hereinafter, referred to as xe2x80x9cEVFxe2x80x9d) 5 including a liquid crystal panel for displaying a photograph captured by the CCD 4, a liquid crystal panel drive circuit and a drive voltage generating circuit.
The CPU 100 comprises a CPU, ROM and RAM and controls various operations in accordance with operation programs stored in ROM. A stroboscopic device 6 comprises a booster circuit and a stroboscopic light emitting unit and is controlled by the CPU 100.
A motor driver DRV 200 comprises transistors Tr11 through Tr14 and diodes D11 through D14 for supplying current to a coil La of a motor 9 for driving members for exposure control such as the shutter or the diaphragm. Current supplied to the coil La is supplied from the battery E via a regulator REG300.
A voltage detecting circuit VDT0 detects the voltage of the battery E and generates an output for detecting battery rundown when the voltage of the battery E becomes proximate to a minimum voltage guaranteeing proper operation of the CPU 100. The CPU 100 generates a low battery alarm at a display unit (not illustrated) in accordance with the battery rundown detected output and prevents the shutter from being released by making a release switch (not illustrated) inoperable.
Generally, when power source voltage is lowered, a control circuit such as the CPU 100 undergoes erroneous operation or becomes inoperable and when a battery is used as a power source, the larger the consumed current, the lower the battery voltage becomes due to the internal resistance of the battery.
According to the example of FIG. 14, in order to avoid operational failure of the CPU 100 due to lowering of the battery voltage, the voltage of the battery E is monitored by the voltage detecting circuit VDT0 and when the battery voltage becomes proximate to the minimum voltage guaranteeing proper operation of the CPU 100, a battery low alarm is displayed and the shutter is prevented from being released. That is, the xe2x80x9cbattery rundownxe2x80x9d is determined at a time point at which the battery voltage becomes proximate to the minimum voltage guaranteeing proper operation of the CPU 100.
According to the above construction having the battery voltage monitoring function, there is a case in which by virtue of the consumed current flowing in a circuit which is temporarily operated, that is, by virtue of a temporary increase in the consumed current, the battery voltage decreases to proximate the minimum voltage guaranteeing proper operation of the CPU 100 and battery rundown is determined although there still is an allowance actually in the capacity of the battery.
Particularly, in the case of a digital still camera, there are many cases where an electronic viewfinder comprising a photographing unit having CCDs or a liquid crystal panel is used, where the battery is used as the power source for such a viewfinder. There is a case in which such a viewfinder is operated in photographing, particularly in determining composition. When the shutter is driven under this state, the consumed current is temporarily increased and accordingly., there is a significant possibility of producing the above-described erroneous determination.
Recently, in order to avoid the above-described drawback in a digital still camera, it has been devised that for example, in charging a stroboscope device, simultaneous drive of a plurality of circuits is avoided as much as possible such that operation of the liquid crystal panel of the electronic view finder is stopped to thereby lower a peak current value of the battery E.
However, even when various circuits are prevented from being driven simultaneously as much as possible, there are still present circuits which must be driven simultaneously in view of the mechanism and in that case, the peak current of the battery E is naturally increased and there is a problem of determining xe2x80x9cbattery rundownxe2x80x9d even with a battery in which only a short time period has elapsed from the start of use.
The present invention has been devised to overcome the forgoing drawbacks. According to an aspect of the invention, there is provided an exposure control drive apparatus comprising a booster circuit for boosting voltage of a battery, a capacitor charged via the booster circuit, an actuator for driving an exposure control member with the capacitor as a power source, a detecting circuit for detecting a value in accordance with a current flowing in the booster circuit, and a control circuit for controlling operation of the booster circuit in accordance with a magnitude of the detected value. Therefore, a magnitude of current flowing from the battery can be controlled. Therefore, for example, a peak value of the current flowing from the battery can be restricted and temporary voltage drop of the battery can be restrained.
When the control circuit controls the operation of the booster circuit such that the detected value does not exceed a predetermined value, for example, the peak value of the current flowing from the battery can be restricted and temporary voltage drop of the battery can be restrained.
It is preferable that the booster circuit includes a booster coil and a switching transistor and outputs a boosted voltage by a switching operation of the switching transistor, and the control circuit controls the switching operation of the switching transistor in accordance with the magnitude of the detected value.
When the control circuit controls a pulse width of a switching pulse specifying the switching operation of the switching transistor in accordance with the magnitude of the detected value, individual pulse widths can be switched and accordingly, a value of the current flowing from the battery can be finely controlled.
The booster circuit may include a resistor in which the on current of the switching transistor flows, and the detecting circuit may detect voltage generated by flowing the on current in the resistor.
When the booster circuit includes a resistor in which on current of the switching transistor flows, and the detecting circuit detects a sum of voltages generated by flowing the on current in the resistor and the saturated voltage of the switching transistor generated when the switching transistor flows the on current, the saturated voltage of the switching transistor can be used as a bias voltage and accordingly, the detection level can be shifted.
There may also be constructed a circuit in which the booster circuit includes a booster coil and a switching transistor and outputs a boosted voltage by a switching operation of the switching transistor, a collector of the switching transistor is connected to one end of the capacitor via a diode, an emitter of the switching transistor is connected to the other end of the capacitor and the detecting circuit detects voltage generated in a resistor provided between the emitter of the switching transistor and a low potential side of the battery. In this case, the detected voltage is increased or reduced retardedly from on or off of the switching transistor by influence of the capacitor and becomes easy to detect.
It is preferable that the duty cycle of the switching pulse is equal to or larger than 50%.
The booster circuit may be prohibited from executing a boosting operation in driving the actuator and/or at a desired time.